The present invention relates to a device for controlling the flow of fluid, in particular hydraulic fluid, from a high-pressure source to a fluid unit, such as storage container or consumer unit or actuator, having an on/off valve substantially without any throttling effect located in a supply conduit, and, more particularly, to a device in which the on/off valve permits a specified or specifiable average supply velocity to be adjusted by cyclic opening and closing with a specified or specifiable ratio between the opening and closing periods.
Fluid flow control devices with on/off valves are basically known and, relative to systems with continuously acting valves, offer the advantage of being more easily achieved in practice. A fundamental disadvantage of such known systems is the relatively high energy requirement. This becomes especially clear when the case of a hydraulic actuator unit (e.g., a piston/cylinder unit), which is connected via the on/off valve to the pressure source in order to carry out an actuation stroke, is considered. In order to carry out an actuation stroke of a specified magnitude, it is necessary to introduce a corresponding quantity V of the hydraulic medium into the hydraulic actuator unit. If the pressure of the pressure source has the value p.sub.o, hydraulic energy E.sub.h =V.p.sub.o is consumed during the introduction of the hydraulic medium into the actuator unit. This hydraulic energy E.sub.h is generally larger than the mechanical work E.sub.m performed by the actuator unit because the pressure p.sub.o is generally clearly greater than the minimum pressure necessary for carrying out the actuation stroke of the actuator unit. If, for example, a mass m has to be raised vertically by a distance x by means of the actuator unit, the mechanical work performed by the actuator unit is represented by the product of the weight of the mass m and the distance x. This product is clearly less, to a greater or lesser extent, than the product V.p.sub.o which represents the consumption of hydraulic work E.sub.h.
Up to now, no easily practical possibilities for reducing the hydraulic power requirements have been indicated. In German Offenlegungsschrift 27 52 899, a hydraulic consumer unit is connected to a pressure source by way of a cyclically switchable on/off valve and a non-return valve located behind it in series; the non-return valve only permits flow towards the consumer unit. A first throttle is located between the non-return valve and the on/off valve. A further throttle connects the consumer-unit side of the on/off valve, and a pressure storage container located there, to a low-pressure reservoir. Using this known arrangement, the flow of hydraulic medium to the consumer unit can be controlled very sensitively.
Hydraulic medium can only flow to the consumer unit of such known system, however, when the ratio between the opening and closing periods of the on/off valve is sufficiently large, i.e. when the opening periods are relatively long compared with the closing periods. When the ratio mentioned is less than a threshold value, the non-return valve leading to the consumer unit remains closed. High throttling losses do, of course, occur and this is so even if the on/off valve operates substantially without any throttling effect.
A circuit arrangement for controlling a hydraulic drive motor with energy recovery during the braking process is described in German Offenlegungsschrift 38 34 918. Controllable throttle valves are located at the inlet and outlet ends of the hydraulic motor, and are used to control the inlet and outlet of hydraulic medium to and from the hydraulic motor. The circuit arrangement includes on/off valves by virtue of which the inlet end of the inlet-end throttle valve of the hydraulic motor is connected, during its acceleration, to a high-pressure source and is connected, during an operation at constant speed, to a pressure source of lower pressure. In addition, the on/off valves are connected to non-return valves such that regenerative braking of the hydraulic motor is made possible, i.e. the outlet end of the outlet-end throttle valve of the hydraulic motor is connected to a high-pressure storage container during braking so that the hydraulic motor, now operating as a pump, introduces hydraulic medium into this storage container. In this way, the kinetic energy of the hydraulic motor and the units drive-connected to it can be used to charge the high-pressure storage container which can then be used subsequently as the high-pressure source during an acceleration of the hydraulic motor.
Systems for the controlled reduction of pressure in displacer units of presses and the like are known from the publication O+P "Olhydraulik und Pneumatik" 34 (1990) No. 4, pages 224 to 231; see, in particular, FIG. 4 on page 226. The displacer working space can be connected by several parallel conduits, which have different throttling resistances and are controlled by on/off valves, to a low-pressure reservoir. In order to ensure a pressure which initially falls slowly in the displacer unit, the conduit on/off valve is initially opened with maximum throttling resistance. It is possible to lengthen the opening periods successively. The on/off valve of a conduit with a lower throttling resistance is then additionally actuated later in a manner similar to that previously described.
The question of how the energy necessarily expended during the introduction of the hydraulic medium into the displacer unit can be reduced is not discussed in the above-mentioned publication. The sole provision is for a non-return valve, which only permits a flow towards the displacer unit, located between the low-pressure side and the displacer unit. This non-return valve is obviously intended to be used to ensure complete filling of the displacer unit when the displacer working space expands.
An object of the present invention is to keep the fluidic energy used for the supply of fluid to a fluid unit as small as possible, in particular when the pressure of a fluidic high-pressure source is large compared with the pressure in the fluid unit.
This object has been achieved, according to the present invention, in a flow control device by locating a non-return valve between the fluid unit and a low-pressure connection or reservoir to prevent flow to this low-pressure connection or reservoir so that dynamic vacuum peaks occurring after the closing of the on/off valve at the fluid unit side cause an additional flow of fluid via the non-return valve.
The present invention is based on the recognition that when the on/off valve is closed, dynamic pressure fluctuations with marked vacuum peaks inevitably occur on the side of the on/off valve leading to the fluid unit. These vacuum peaks can then be used for an additional flow of fluid via the non-return valve. This causes, on one hand, a smoothing of the pressure fluctuations while, on the other hand, no additional external power is consumed for the additional supply of fluid.
The present invention therefore makes it possible to use the kinetic energy generated by the fluid flowing when the on/off valve is open or the associated inertia effects and pressure fluctuations, i.e. in general terms, the inductance of the system, for the supply of fluid to the fluid unit.
The system according to the present invention operates particular effectively if, in accordance with a preferred embodiment, the outlet side of the non-return valve is connected to a conduit part or branch leading to the fluid unit, in which conduit part or branch high flow velocities occur when the on/off valve is open. This is because the high flow velocities cause strong inertia effects when the on/off valve is closed and, correspondingly, a strong flow of fluid via the non-return valve.
If necessary, it is possible to provide for the low-pressure connection or the low-pressure reservoir to have a pressure which is, in fact, reduced relative to the high-pressure source but the reservoir is still not unpressurized. This measure is advantageous for avoiding cavitation.